Thermo-optical devices are known, e.g., from the description given by Diemeer et al. in Journal of Lightwave Technology, Vol. 7, No. 3 (1989), 449-453. Their working is generally based on the phenomenon of the optical waveguide material employed exhibiting a temperature dependent refractive index (polarisation independent thermo-optical effect). Such devices have been realised, int. al., in inorganic materials such as ion-exchanged glass and titanium-doped lithium niobate. An advantage of the use of all-polymeric waveguides for thermo-optical devices disclosed by Diemeer et al. consists in that a modest increase in temperature may result in a large index of refraction change. The device described by Diemeer is an all-polymeric planar switch. Switching is achieved by employing total internal reflection from a thermally induced index barrier. The device comprises a substrate (PMMA), a waveguiding structure (polyurethane varnish), and a buffer layer (PMMA), with the heating element being a silver stripe heater deposited by evaporation upon the buffer layer through a mechanical mask.
A thermo-optical switching device has also been disclosed by Mohlmann et al. in SPIE Vol. 1560 Nonlinear Optical Properties of Organic Materials IV (1991; ), 426-433. Use is made of a polymer in which a waveguide channel can be created through irradiation. The disclosed device is a polarisation/wavelength insensitive polymeric switch comprising an asymmetric Y-junction. The switching properties are based on heat-induced refractive index modulations causing variations in the mode evolution in such asymmetric Y-junctions. The device comprises a glass substrate and a polymeric multilayer comprising an NLO polymer. Another thermo-optical device disclosed is a thermo-optically biassed electro-optic Mach Zehnder interferometer.
In Electronic Letters, Vol. 24, No. 8 (1988), 457-458 an optical switch is disclosed in which optical fibers are coupled using a single-mode fused coupler having a silicone resin cladding material applied over the coupling region. Switching is achieved by a thermally induced refractive index change of the silicone cladding.
In U.S. Pat. No. 4,753,505 a thermo-optical switch is described comprising a layered waveguide in which the material having a temperature dependent refractive index is a polymer or glass.
In U.S. Pat. No. 4,737,002 a thermo-optical coupler is described which may be formed using either optical fibers or integrated optics.
In EP-A-0 642 052 and Granestrand et al., "Integrated Optics 4.times.4 Switch Matrix With Digital Optical Switches," Electronics Letters, Jan. 4, 1990, Vol. 26, No. 1, pp. 4-5, a cascade or tree structure of 1.times.2 optical switches is disclosed.
In GB-A-225980 discloses a network comprising two 1.times.2 optical switches, wherein the input of one of the switches is optically connected to one of the output of the other switch.
While the disclosed polymeric thermo-optical devices sufficiently establish that thermo-optical effects can be employed to achieve, e.g., switching, in the known devices the extinction (defined as: 1010 log= (optical power on)/optical power off) leaves much to be desired and crosstalk is often a problem. These problems can be solved by providing a thermo-optical device with a waveguiding structure of a specific design.